Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
FIG. 1 illustrates a fixing device 9R installed in such image forming apparatuses, which includes a fixing roller 91R and a pressing roller 92R that apply heat and pressure to a recording medium P bearing a toner image. For example, the pressing roller 92R heated by a halogen heater 98R disposed inside the pressing roller 92R is pressed against the fixing roller 91R heated by a halogen heater 97R disposed inside the fixing roller 91R to form a fixing nip N therebetween through which the recording medium P bearing the toner image is conveyed. As the fixing roller 91R and the pressing roller 92R rotate and convey the recording medium P in a recording medium conveyance direction C1 through the fixing nip N, the fixing roller 91R and the pressing roller 92R apply heat and pressure to the recording medium P, melting and fixing the toner image on the recording medium P. The fixing roller 91R is constructed of a metal pipe 93R and a rubber layer 95R coating the metal pipe 93R. Similarly, the pressing roller 92R is constructed of a metal pipe 94R and a rubber layer 96R coating the metal pipe 94R. Since the rubber layers 95R and 96R work as a thermal resistor, the rubber layers 95R and 96R have a small thickness that creates a short fixing nip length in the recording medium conveyance direction C1. Accordingly, the fixing roller 91R and the pressing roller 92R that form the short fixing nip length may apply heat and pressure to the recording medium P insufficiently when the recording medium P is conveyed at high speed, resulting in faulty fixing.
To address this problem, a fixing device 10R incorporating a fixing belt 7R that creates a longer fixing nip length is proposed as shown in FIG. 2. For example, the fixing belt 7R is stretched over a fixing roller 4R and a heating roller 8R. A pressing roller 2R is pressed against the fixing roller 4R via the fixing belt 7R to form a fixing nip N between the pressing roller 2R and the fixing belt 7R. As a recording medium P is conveyed through the fixing nip N in a recording medium conveyance direction C2, the fixing belt 7R heated by a heater 5R via the heating roller 8R and the pressing roller 2R heated by a heater 6R apply heat and pressure to the recording medium P, melting and fixing the toner image on the recording medium P. Since the fixing roller 4R incorporates a thick rubber layer, the fixing roller 4R creates the longer fixing nip length in the recording medium conveyance direction C2. However, in order to attain the longer fixing nip length, the fixing roller 4R needs to have a greater loop diameter that decreases the curvature of the fixing roller 4R and the fixing belt 7R stretched over the fixing roller 4R, hindering separation of the recording medium P from the fixing belt 7R after it is discharged from the fixing nip N. Accordingly, a separator 3R that contacts the fixing belt 7R to separate the recording medium P from the fixing belt 7R is required at the exit of the fixing nip N, upsizing the fixing device 10R.
To address the drawbacks of the fixing device 10R shown in FIG. 2, a fixing device incorporating a stationary pad instead of the fixing roller 4R is proposed. For example, the stationary pad has a substantially rectangular shape in cross-section that increases the fixing nip length and at the same time provides a greater curvature that facilitates separation of the recording medium from a fixing belt stretched over the stationary pad without the separator 3R shown in FIG. 2. However, the stationary pad is too small to endure high pressure from a pressing roller pressed against the stationary pad via the fixing belt, failing to form the uniform fixing nip.
Recently, the image forming apparatuses are requested to form a high quality color toner image on a recording medium at high speed. In order to fix the high quality color toner image on the recording medium, a higher pressure and a longer nip time for which the recording medium bearing the toner image is conveyed through the fixing nip to receive heat and pressure are required. Accordingly, a longer fixing nip length is required to allow the recording medium to be conveyed through the fixing nip for the longer nip time even at high speed. However, as described above, the component (e.g., the fixing roller 4R depicted in FIG. 2) that creates the longer fixing nip length and at the same time endures the higher pressure from the pressing roller 2R may have the great size that decreases its curvature, thus hindering separation of the recording medium P from the fixing belt 7R after the recording medium P is discharged from the fixing nip N. Conversely, the component (e.g., the stationary pad) that creates the longer fixing nip length and at the same time provides the greater curvature may not endure the higher pressure from the pressing roller. Additionally, the fixing belt 7R looped over the fixing roller 4R or the stationary pad that creates the longer fixing nip length may slip over the fixing roller 4R or the stationary pad, resulting in faulty conveyance of the recording medium P passing through the fixing nip N and ineffective heat conduction from the heating roller 8R to the fixing belt 7R.
Accordingly, there is a need for a technology that attains the higher pressure and the longer fixing nip length at the fixing nip N required to form a high quality color toner image as well as the greater curvature required to separate the recording medium P from the fixing belt 7R without slippage of the fixing belt 7R.